The invention concerns a device for straightening a curved steel strand cast continuously by means of a casting wheel machine or a curved mold continuous casting machine. The invention particularly concerns such a device where the strand is passed between rolls such as straightening- bending- guide- or counter-rolls and/or roll pairs to provide a bending moment to the strand.
The shaping of solids, their deformation, and thus their bending behavior can be predicted with mathematical accuracy only within the range of validity of Hooke's Law. This ideal elastic behavior of a body exists only under conditions where the hysteresis curve of the elastic material evidences an area approaching or in close proximity to zero. Thus, Hooke's Law is valid for elastic solids only in the range of low forces and correspondingly minor deflection.
With increasing deflection in the quasi-elastic range, considerable substitution and boundary conditions, restricted to the case in point, have to be taken into account for mathematically determining the material behavior if one wishes to apply Hooke's Law to an even limited extent. With greater deformation or application of force in the yield region, the physical laws of solids no longer apply. Here, even if only inadequately and again with specification of special boundary conditions, only the knowledge and mathematical combination from fluid continuum physics can be applied. The Plastic region, directly before fracture of the solid, can still only be determined empirically for the practical technical problems which usually occur.
Even less clear are the conditions in continuous casting, especially in curved mold casting, as here bending forces have generally to be applied to a strand of material which can evidence fully elastic properties along its outer solidified skin, while, at least in its inner region, it is still subject to the laws of deformation of fluids, and in the transition zones between solid and fluid behavior also evidences plastic and/or quasi-plastic behavior patterns.
In the case of continuous casting plants with vertical bending configuration, where the strand emerging vertically from the mold is cooled in vertical travel until it has completely solidified, it is still possible to a large extent to calculate the path of the strand and thus the necessary positioning of the bending rolls and straightening rolls to apply relationships from Hooke's Law in this situation, only minor deformation has to be carried out by means of relatively low forces with large bending radii occurring on a strand of material which already possesses largely elastic properties.
Particularly with casting wheels, but also with certain curved molds of small radius, calculation of an ideal curve path within the subsequent bending zone leading up to the straightened steel strand is more difficult. With adequately elastic strand behavior, physical laws are resorted to for calculation of the curve. The curve path is known from the study of the loading of a unilaterally secured beam and also from uniform loading of a beam secured at both ends. In accordance with the two last mentioned boundary cases, conditions are then created for positioning of straightening and bending rolls and, if necessary, corresponding counter rolls which are based extensively on these two mathematical model tests.
Known practice for straightening continuously cast steel from a casting wheel has previously consisted of designing bending and straightening rolls applied at the bending moment based on the model of the unilaterally secured beam.
It is also known to transmit at least two bending moments to a strand emerging from a continuous steel casting plant by means of two rolls pairs positioned at a distance from one another as well as with force-transmitting roll pairs for downward deflection of the emerging strand (DE-AS 23 41 563). This known form of design is distinguished in the area of the bending zone by a number of rolls guiding the strand positively between them, such that the torque imparted to the strand by the roll pairs mentioned cannot lead to free flexure between them. In fact, the positive guidance provided by the number of roll pairs located in between defines a positively prescribed bending curve. This bending curve creates a change in elongation of the strand at the maximum of its crack-free progress beginning and ending at zero, and does not exceed the value of 0.0025%/mm in bending and 0.0030%/mm in straightening.
With all known strand guide systems, straightening is carried out progressively in the bending zone. In a strand guide system, the bending and straightening is carried out on a strand which has a core which is still molten and has a relatively thin strand shell, the radius of curvature being gradually increased in several stages. As in bending and straightening, the progress of change in elongation is important. Elongation can lead to cracks. If empirically determined maximum values are exceeded. The cracking attributable in particular to the fact that with steel in the transition phase form its molten to its solid aggregate state, resistance to a deformation is dependent on the rate of deformation. A mathematical statement is thus possible at least from the point of view of the problem definition.
Seen from the aspect of the possibility of such a mathematical statement of an optimum bending curve, becomes even more intricate if in high temperature ranges above about 1000.degree. C. Additionally, if the solidifying outer skin of the strand has not yet changed in defined manner from a quasi-plastic to a quasi-elastic state, the cast strand cannot be straightened with a small radius. Straightening trains in such high temperature ranges and at so early a state of continuous casting are, for example, to be seen when a casting wheel plant or a continuous casting plant with curved mold is to be operated in direct conjunction with a rolling mill, to minimize energy consumption in the process cycle. For this purpose it is necessary that the temperature of the cast strand be kept as high as possible, but on the other hand, at excessively high temperatures defined processing options can hardly be maintained. The strand material, undefined in respect of its stress behavior, starts in these regions to flow even with minor stress loading and to behave like an incompressible fluid rather than a solid subject to the laws of rigid continuum mechanics.
If the temperature range concerned here one applies customary straightening processes using bending rolls, straightening rolls and counter rolls to a curved cast steel strand, it can be seen that the soft strand material begins to flow perceptibly before reaching the straightening roll set-up to such a degree that it sags visibly in this region thus instead of achieving the desired gradual increases in bending radius pronouncedly curved sections are detectable, even by comparison with even the initial curvature. The previously customary geometrical designs and assumed conditions within a bending zone require improvement, where the bending radius merges tangentially into the outfeed straight line. If the said circular arc is flattened but double flexure unfavorably affecting stress conditions in the strand or even overflexure of the strand is observed before reaching the straightening roll, it is necessary or desirable to deviate from conventional practice to a technical compromise. Additional overflexure will of necessity lead to significant increase in the risk of cracking and thus to reduced quality.
This is where the present invention comes into its own, because it is based on the technical problem of guiding the curved cast strand in a process of the class in question, such that double flexure is prevented, ensuring minimization of the flow rate and of the stresses exerted on the high temperature strand.